This shared instrumentation grant application seeks funds to purchase a Biacore 3000 biosensor intsrument. The Biacore 3000 uses surface plasmon resonance to monitor biomolecular interactions in real time. The major users (75%) of this fully automated, sensitive, state-of-the-art instrument will be six investigators in the Department of Biochemistry and Molecular Biology in the Johns Hopkins University Bloomberg School of Public Health. The specific projects of these users cover a broad range of biomedically important areas: 1) signaling by polyubiquitin chains in proteolysis and DNA repair (Dr. Pickart); 2) involvement of the DnaJ and DnaK molecular chaperones in substrate selection and nucleoprotein disassembly reactions (Dr. McMacken); 3) regulation and function of SUMO modification (Dr. Matunis), 4) interactions of repair proteins with alkyl interstrand cross-linked DNA (Dr. Miller); 5) DNA repair by Ku-dependent DNA end-joining (Dr. Hanakahi); and 6) RNA splice site choice by U2 auxiliary factor (Dr. Kielkopf). A significant fraction (25%) of the time on the Biacore 3000 will be set aside for other researchers in our Department and other departments at Johns Hopkins University. The availability of this instrument will strongly promote multiple NIH-funded research programs at the Johns Hopkins University. In recognition of this fact, the Bloomberg School of Public Health has made a substantial commitment to the long-term operation and maintenance of the Biacore 3000 [unreadable] [unreadable]